1. Field of the Invention
The present invention relates generally to radiation beam geometry evaluation, and finds application in optical recording and playback systems. In particular, the present invention relates to arrangements used to monitor the focus position of a light beam with respect to an optical disk.
2. Background of the Invention
Optical recording and playback systems, such as those utilizing optical memory disks, compact disks, or video disks, require precise focusing of an illuminating optical beam through an objective lens onto the surface of an optical disk. The incident illuminating beam is generally reflected back through the objective lens, and is then used to read information stored on the disk. Subsequent to passing back through the objective lens, a portion of the reflected beam is typically directed to an apparatus designed to gauge the focus of the illuminating beam on the disk. Information extracted from the reflected beam by this apparatus may then be used to adjust the focus of the illuminating beam by altering the position of a movable objective lens relative to the disk.
A number of techniques for detecting the focus of an illuminating optical beam are known. For example, U.S. Pat. Nos. 4,423,495, 4,425,636 and 4,453,239 employ what has been termed the "critical angle prism" method of determining beam focus. In this method an illuminating beam reflected from a storage disk is made incident upon a detection prism surface which is set very close to a critical angle with respect to the reflected illuminating beam. When the focus of the illuminating beam on the surface of the disk deviates from a desired state, the variation in the amount of optical energy reflected by the detection prism surface may be used to derive a focus error signal used to adjust the focus of the illuminating beam.
The critical angle prism method generally requires that the orientation of the detection prism surface relative to the reflected illuminating beam be precisely adjusted. This requirement arises as a result of discontinuities in the reflectivity characteristic of the detection prism in the neighborhood of the critical angle and makes focus error detection systems based on this method extremely sensitive. However, the critical angle technique has several disadvantages. First, the focus error signal it produces depends on the light reflection at the interface between the detection prism surface and air; thus, changes in altitude, which change the index of refraction of the air, can cause false focus readings (offsets) to occur. Also, the critical angle technique is inherently unsuitable for use in differential focus sensing systems.
Differential systems are increasingly important because they allow cancellation of certain types of noise that can occur in optical disk drives. The critical angle method is unsuited to differential operation for two reasons. First, the transmitted beam produced by the sensing prism is compressed along one axis, making it unsymmetrical with the reflected beam. Symmetry of the two beams is preferred in a differential system to optimize the noise-cancellation properties in varied environments. Second, at the point on the reflectivity curve of a critical angle prism where the intensities of the two beams are balanced, the slope is far too low to produce a useful differential focus error signal.
A focus detecting apparatus which requires somewhat less precise adjustment of the optical surface on which the reflected illuminating beam is incident, when compared to the critical angle technique is disclosed in U.S. Pat. No. 4,862,442. In particular, the optical surface described therein comprises a dielectric multilayer coating having a reflectivity which varies continuously with respect to the angle of incidence of the reflected illuminating beam. It follows that rotational maladjustment of the surface comprising the multilayer coating will have smaller effect on the value of the focus error signal, but that also the technique will have reduced angular sensitivity. Also, inaccuracies in the focus error signal produced by multilayer dielectric systems may develop in response to relatively slight changes in the wavelength of the reflected illuminating beam. Such sensitivity to wavelength changes is undesirable since the focus error signal is designed to relate solely to the focus of the illuminating beam.
In addition, certain systems using a dielectric multilayer reflecting surface provide focus error signals having only a limited degree of sensitivity. For example, FIG. 2 of U.S. Pat. No. 4,862,442 shows a particular reflectivity characteristic for a layered dielectric reflecting surface, with the slope of the reflectivity characteristic being proportional to the sensitivity of the focus error signal. The disclosed reflected intensity ranges in value from approximately 0.75 to 0.05 over angles of incidence extending from 42 to 48 degrees. This reflectivity change of approximately 10% per degree produces a focus error signal of relatively low sensitivity.
Accordingly, a need in the art exists for an optical arrangement characterized by a reflectivity profile which allows generation of a highly sensitive focus error signal relatively immune to changes in altitude and to chromatic aberration, and which is capable of use in differential systems.